[PATCH] sysctl: do not allow a 64bit value write in a 32bit knob

[Aristeu Rozanski]

Writing to a sysctl file that uses proc_dointvec_minmax like 
user/max_uts_namespaces
a larger than 32 bit value won't cause an error as expected but instead will 
zero
its value:
        # echo 214748364800000 > max_uts_namespaces
        # cat max_uts_namespaces
        0

This patches fixes it.

Signed-off-by: Aristeu Rozanski <a...@redhat.com>
Cc: "Luis R. Rodriguez" <mcg...@kernel.org>
Cc: Kees Cook <keesc...@chromium.org>

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 4ac9b9a..243f277 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2486,7 +2486,8 @@ static int do_proc_dointvec_minmax_conv(bool *negp, 
unsigned long *lvalp,
        if (write) {
                int val = *negp ? -*lvalp : *lvalp;
                if ((param->min && *param->min > val) ||
-                   (param->max && *param->max < val))
+                   (param->max && *param->max < val) ||
+                   *lvalp >> (sizeof(int) * 8))
                        return -EINVAL;
                *valp = val;
        } else {